Fix indented container directives

Closes GH-1.

lib/tokenize-directive-container.js

@@ -104,7 +104,13 @@ function tokenizeDirectiveContainer(effects, ok, nok) {
   }
 
   function chunkStart(code) {
-    var token = effects.enter('chunkDocument', {
+    var token
+
+    if (code === null) {
+      return after(code)
+    }
+
+    token = effects.enter('chunkDocument', {
       contentType: 'document',
       previous: previous
     })

test.js

@@ -1020,6 +1020,30 @@ test('micromark-extension-directive (syntax)', function (t) {
       'should support a thematic break before a container'
     )
 
+    t.equal(
+      micromark(' :::x\n ', options({'*': h})),
+      '<x></x>',
+      'should support prefixed containers (1)'
+    )
+
+    t.equal(
+      micromark(' :::x\n - a', options({'*': h})),
+      '<x>\n<ul>\n<li>a</li>\n</ul>\n</x>',
+      'should support prefixed containers (2)'
+    )
+
+    t.equal(
+      micromark(' :::x\n - a\n > b', options({'*': h})),
+      '<x>\n<ul>\n<li>a</li>\n</ul>\n<blockquote>\n<p>b</p>\n</blockquote>\n</x>',
+      'should support prefixed containers (3)'
+    )
+
+    t.equal(
+      micromark(' :::x\n - a\n > b\n :::', options({'*': h})),
+      '<x>\n<ul>\n<li>a</li>\n</ul>\n<blockquote>\n<p>b</p>\n</blockquote>\n</x>',
+      'should support prefixed containers (4)'
+    )
+
     t.end()
   })